It has been suggested recently that the two most widely used reference strains of Candida albicans (NCPF 3153 and B311) were derived from the same parental strain. However, records maintained at the PHLS Mycological Reference Laboratory suggest otherwise. In an attempt to resolve this contradiction, assessments were made of biotypes, resistogram patterns, morphotypes and restriction fragment length polymorphisms (RFLP) for nine isolates of NCPF 3153 and six of B311, obtained from 11 different centres. Biotyping results showed that with one exception (isolate No. 3, putatively derived from NCPF 3153), only strains derived from B311 were resistant to flucytosine at 12.5 micrograms ml-1 in agar. Moreover, strain No. 3 differed from other NCPF 3153 derivatives in its resistogram and RFLP patterns, which were indistinguishable from B311 isolates. It was concluded that the two reference strains were not derived from the same parental strain, and that they could be distinguished from each other by biotype, resistogram and RFLP patterns. Strains designated as B311 resembled each other closely, in contrast to NCPF 3153 isolates, which had variable resistogram and RFLP patterns. The simplest screening procedure for distinguishing B311 isolates was their resistance to flucytosine. Since test isolate No. 3 differed consistently from other putative NCPF 3153 isolates, it was concluded that its status was uncertain and that its designation as = NCPF 3153 should be discontinued.